1. Field of the Invention
The present invention relates to a new monomer and polymer used in making a photoresist, and a photoresist composition comprising the same. More specifically, it relates to a new monomer of a lithocholylacrylate type and introducing the new monomer to a polymer to form a photoresist. The photoresist preferably is used in a lithography process using a KrF(248 nm) or an ArF(193 nm) light source, which are light sources that can be used in the manufacture of 1G or 4G Dynamic Random Access Memory ("DRAM") semi-conductor integrated circuits.
2. Description of the Prior Art
In general, a photoresist resin which is to be used with an ArF source should have suitable etching resistance and adhesiveness, with low light absorption at 193 nm wavelength. Such a photoresist resin typically is developable by using about 2.38 wt % aqueous tetramethylammonium hydroxide(TMAH) solution. However, these types of photoresist resins are difficult to synthesize. Up to now, many researchers have focused their studies on novolac type resins to increase transparency at 193 nm wavelength and increase etching resistance. As a part of this research, a three component resist system has been suggested by Bell laboratory. The resist system suggested by Bell Labs uses a lithocholic ester compound as a dissolution inhibitor and a copolymer resin represented by the following Formula I, wherein the copolymer resin has, norbornene acrylate and maleic anhydride substituents in the main chain: ##STR3##
The lithocholic ester compound used in the aforementioned resist system has excellent etching resistance and high transparency at far-ultraviolet rays, particularly at 193 nm. However, when too much of the lithocholic ester compound is included in the resist composition, for example 40 wt % or more, a residue is extracted by crystallization.
As one skilled in the art will appreciate, flow of the resist typically occurs during a heat-treating process such as pre-bake process or post-bake process, which lowers the free transition temperature(Tg) of the lithocholic ester compound; for example to about 100.degree. C. and below. Thus, if the resist contains a residue from the lithocholic ester compound, the photoresist pattern may be distorted, thereby adversely affecting the resolution of the etching process.